1. Field of the Invention
The present invention relates to a sheet processing apparatus and an image forming system having the sheet processing apparatus, such as a copying machine, a printer, and a facsimile.
2. Description of the Related Art
In the related art, an image forming apparatus using an electrophotographic system develops a latent image formed on a photosensitive drum as an image bearing member to visualize it. This visualized image (toner image) is transferred onto a sheet using an electrostatic force. Then, the toner image on the sheet is fixed by applying heat and pressure so that an image is recorded and formed on the sheet.
As a fixing device for such an image forming apparatus, a heat roller fixing method is employed, in which a heat source such as a heater is internally provided, and a fixing nip portion is formed with a fixing roller maintained at a predetermined temperature and an elastic pressure roller making pressed contact with the fixing roller, so that a toner image is fixed onto a sheet in the fixing nip portion.
In recent years, in an image forming apparatus (particularly, a full-color image forming apparatus) including such a type of the fixing device, there is known a fixing device capable of increasing a heating time and increasing a fixing speed in order to improve a chromogenic property or image quality of a toner image. For example, as discussed in Japanese Patent Laid-Open No. 5-150679, there is known a so-called belt nip type fixing device in which an endless fixing belt stretching around a plurality of rollers makes pressed contact with a heating roller.
In addition, in recent years, there is a demand for a high process speed in order to obtain a high-speed output image forming apparatus. For this purpose, it is necessary to provide a wider nip in a width direction perpendicular to a sheet feeding direction. In addition, a belt fixing method has been proposed and commercially produced, in which a wider nip width is obtained by substituting one or both of the fixing roller and the pressure roller with an endless belt.